1. Field of the Invention
The present invention relates to a digital image pickup apparatus and, more particularly, to a digital image pickup apparatus which quickly generates a high-quality image with a smaller number of pixels than that of an image sensing device mounted in the image pickup apparatus.
2. Description of the Related Art
Along with the recent rapid spread of personal-computers, the demand for digital cameras serving as image input devices is increasing. In addition, high-quality recording apparatuses such as digital video recorders are widely used as video recorders.
The image quality of an electronic still camera is decided by several factors. Especially, the number of pixels of an image sensing element is a very important factor for the resolution of a sensed image. Some recent commercially available electronic still cameras have more than 5 million pixels. However, data of 5 million pixels is not always necessary for all application purposes. Images displayed on the Webs of the Internet often have, if anything, smaller pixel sizes.
In current digital cameras, the flashing time from the image sensing element to the image memory is a bottleneck. Most models having a large number of pixels cannot execute high-speed continuous shooting. In addition, even digital cameras need to have a video sensing function as an additional function. Hence, transfer to the memory must be done at a high speed. For this purpose, the data amount to be processed is preferably reduced in advance.
When the number of pixels of an output image is smaller than that of the image sensing element, the number of pixels to be used is limited in advance. Accordingly, the amount of data to be transferred from the image sensing element to the memory can be reduced, and the memory transfer rate can be increased.
In size reduction by linear interpolation, an image having a large size is generated by using all pixels. Then, an image with a small size is generated by linear interpolation.
FIG. 15 shows a state in which a full-color reduced image is generated from a Bayer matrix by bilinear interpolation sampling. In bilinear interpolation shown in FIG. 15, full-color data of points A, B, C, and D are calculated from R, G, and B data of 12 neighboring points. More specifically, the full-color data of the point A is obtained by linear combination of R43, R45, R63, R65, B34, B36, B54, B56, G44, G53, G55, and G64. Similarly, the full-color data of the point B is obtained by linear combination of R03, R05, R23, R25, B14, B16, B34, B36, G15, G24, G26, and G35. This also applies to the points C and D.
FIG. 16 shows a state in which a full-color reduced image is generated from a Bayer matrix by bicubic interpolation sampling. In bicubic interpolation shown in FIG. 16, full-color data of a point is calculated from R, G, and B data of 48 neighboring points. For example, the full-color data of the point B is obtained by linear combination of R61, R63, R65, R67, R01, R03, R05, R07, R21, R23, R25, R27, R41, R43, R45, R47, B72, B74, B76, B70, B12, B14, B16, B10, B32, B34, B36, B30, B52, B54, B56, B50, G75, G04, G06, G13, G15, G17, G22, G24, G26, G20, G33, G35, G37, G44, G46, and G55.
Such resizing by linear interpolation can ensure a high image quality. However, since linear interpolation is executed by using all pixel data, the arithmetic amount is large. Hence, this method is inappropriate for the above-described continuous shooting function or video sensing.
There is a method of reducing the data amount of memory read, in which an integration function is added to the image sensing element so that a reduced image is generated by reading out a small number of averaged data. Jpn. Pat. Appln. KOKAI Publication No. 2001-245141 discloses a high-speed image reducing method using this method.
Jpn. Pat. Appln. KOKAI Publication No. 2001-016441 discloses an apparatus which executes data thinning and also corrects distortion of data when the number of resolutions is limited. An embodiment of this reference discloses creation of 400-dpi data by an apparatus having a resolution of 600 dpi. When 600-dpi data is directly thinned out, the data is distorted. To cope with this, pixel data to compensate for the distortion of positions is generated from the 600-dpi data by linear interpolation.
The method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-245141 is effective for reduction at a reduction ratio of about 20% or less. In reduction at a high reduction ratio (of about 40% or more), however, the distortion of an image by the readout pixel positions cannot be removed only by averaging by integration. It is therefore difficult to form a high-quality image while changing the size at a reduction magnification in a wide range.
The apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-16441 creates 400-dpi data by executing interpolation totally using 600-dpi data obtained by scanning. The present invention discloses a new technique related to interlaced scanning which copes with a case wherein data of a basic resolution (in Jpn. Pat. Appln. KOKAI Publication No. 2001-16441, 600-dpi data obtained by scanning) cannot totally be read because of a limitation on the read time. For example, a data array free from distortion is created from a thinned data array as shown in FIG. 6(b) in Jpn. Pat. Appln. KOKAI Publication No. 2001-16441.